Thermal head

Abstract: In a thermal head consisting of one or more substrates, each of the substrates comprises a heating element (16), common electrodes (12) arranged on one of the sides of the heating element (16), and individual electrodes (14) arranged on the other side of the heating element. In the vicinity of the side edges of the substrate, the patterns of the two types of electrodes extend from the heating element (16) toward the upper edge and lower edge of the substrate corresponding to the top side and bottom side of the trapezoid, respectively, in such a manner as to be directed at an angle toward the central portion of the substrate. This enables a plurality of substrates to be manufactured with the same electrode patterns, and all the substrates to have the same data transfer direction to simplify the wiring.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a thermal head applicable to output terminals such as printers, plotters, facsimiles and, more particularly, to patterns of electrodes for supplying heating signals to heating elements juxtaposed on a substrate. Description of the Related Arts A typical thermal head consists of a single relatively small-sized substrate having thereon a heating element, common electrodes, and individual electrodes. Through the electrodes, the heating element is supplied with electrical power to generate a heat. Thermal paper or ink film is brought into contact with the heating element by way of a protective film. On the contrary, there is a recently increasing demand for a large-sized thermal head for use in the output terminals such as a CAD system. The large-sized thermal head is preferably composed of a single thermal head substrate. In this case, however, the heating elements or electrodes free from any defect must be formed over a large area of the substrate. The manufacture of such large-sized substrate will therefore face a large technical difficulty, which is not practical. In the prior art, therefore, a plurality of small-sized substrates were arranged to constitute a large-sized thermal head, as disclosed in Japanese Patent Laid-open Pub. No. 2-72967. This will be described in detail with reference to Figs. 1 and 2. Fig. 1 shows, by way of example, a single thermal head consisting of three transversely alternately arranged substrates including substrates 10a and 10c having, in plan view, a substantially trapezoidal shape, and a substrate 10b having an inverted trapezoidal shape. Fig. 2 is an enlarged view of the thermal head depicted in Fig. 1. On the substrates 10a, 10b, and 10c, alternately arranged are comb-teeth-like common electrodes 12 and also comb-teeth-like individual electrodes 14 on which a belt-like heating element 16 extends. The heat is generated in the region of the heating element 16 intervening between the common electrodes 12 and the individual electrodes 14 adjoining to each other. The edges of the substrates 10a, 10c and the substrate 10b adjoining to each other correspond to the oblique sides of the trapezoid of each substrate. The edges of the substrates 10a, 10c and 10b (hereinafter, referred to as side edge portions of the substrate) are formed at predetermined angles 3 and 3' making supplementary angles other than 90 degrees. Moreover, the substrates 10a and 10c are displaced from the substrate 10b by a distance L with respect to the paper feed direction to thereby prevent the heating element 16 from being discontinuous between the adjacent substrates. At the right and left side edge portions of the substrate, the electrodes 12 and the individual electrodes 14 are both disposed to form a predetermined angle 5' in a counterclockwise direction or a predetermined angle 5 with respect to the direction orthogonal to the longitudinal direction of the substrate on which the heating element 16 is arranged. The angles of the electrode patterns 5' and 5 are substantially equal to the angles 3' and 3 of the side edge portions of the substrates so that the electrodes can be disposed for printing even at the side edge portions of the substrate having, in plan view, a substantially trapezoidal shape or an inverted trapezoidal shape. In the case where a single thermal head is constituted of three alternately arranged substrates 10a, 10b, and 10c of a substantially trapezoidal shape or inverted trapezoidal shape having electrode patterns as shown in Figs. 1 and 2, there is the inconvenience that at least two types of electrode patterns are required, that is, one for the trapezoidal substrates 10a and 10c, and the other for the inverted trapezoidal substrate 10b. To this end, at least two systems of photo masks are needed for manufacturing three substrates to constitute a single thermal head, which will lead to increased development cost and development time. Furthermore, producing, cutting and assembling steps are necessary for two types of substrates, which will add to the number of steps. Providing that a thermal head including a combination of two types of substrate having a trapezoidal shape and an inverted trapezoidal shape is formed with only one electrode pattern as in the substrates 10a and 10c, the positions where the common electrodes 12 are arranged on the substrates 10a and 10c will be where the paper arrives earlier than the heating element 16 with respect to the paper feed direction F, while the positions of the common electrodes 12 to be arranged on the substrate 10b will be where paper arrives later than the heating element 16, as shown in Fig. 3. Therefore, when these substrates are employed to constitute a single thermal head, the positions of the common electrodes 12 and the individual electrodes 14 to be arranged with respect to the paper feed direction F will differ for each of the adjacent substrates and hence the direction of the data transfer will differ for each of the substrates, which results in complicated wiring and a complicated structure of the thermal head. Unit heads for divisional type thermal heads as disclosed in EP-A-0 512 251 have a longitudinal array of heating dot portions each of which is connected to wiring electrodes. These wiring electrodes have oblique portions extending from the heating dot portions to the central portion of the substrate. These oblique electrode portions are connected to the heating dot portions by connecting portions which are perpendicular to the row direction of the heating dot portions and to contact portions near the longitudinal side edges of the substrate. This contact portions are perpendicular to the longitudinal direction. Due to this arrangement of the wiring electrodes a protective layer can be provided with V-shaped cutouts to improve the cutting of the material head into unit heads. This cutting is performed perpendicular to the longitudinal direction of the heating area. A thermal head as disclosed in EP-A-0 410 486 has heating resistors connected to each other by an intermediate electrode and arranged like a "greater than" sign ">". This shape is independent from the location of the heating resistors near the left or right end of the row of heating sections and from a cutting direction of the thermal head. In addition, the heating resistors are connected to lead electrodes corresponding to the common and individual electrodes which are perpendicular to the row direction of the heating sections. Another thermal head as disclosed in JP-A-62 042 860 has heat generating resistors which correspond substantially to the heating dot portions shown in EP-A-0 512 251. However, to make it possible to reduce the distance between the heat generating resistors at a connection end part of two substrates the heat generating resistors adjacent to the side edges of the adjacent substrates having enlarged portions toward the side edges as shown in the figure. The cutting direction is not correlated with inclined wiring electrodes. SUMMARY OF THE INVENTION The present invention was conceived to solve the above problems, and the object of this invention is to provide a thermal head capable of being constituted of a plurality of substrates with the same electrode patterns irrespective of the shape of the substrates and having the same data transfer direction for all the substrates, so that it is possible to simplify the wiring. This object is accomplished by a thermal head as described in claim 1. Further developments and improvements of the inventive thermal head are described in the subclaims. According to the present invention, the common electrodes are disposed at predetermined angles with respect to the direction orthogonal to the longitudinal direction of the heating element while the individual electrodes are disposed at predetermined angles in the opposite direction to that of the common electrodes in such a manner that the electrode patterns of the common electrodes and the individual electrodes on the substrate extend from the heating element toward the edges of the substrate corresponding to the top and bottom sides of the trapezium (referred to as upper and lower edges of the substrate) so as to be angled toward the central portion of the substrate in the vicinity of the edges of the substrate corresponding to the oblique sides of the trapezium (referred to as side edges of the substrate). Thus, the common electrodes and the individual electrodes bend around the heating element to present the configuration of a mathematical sign of inequality "<" at the left edge portion of a substrate cut into a desired shape such as a trapezium or an inverted trapezium while presenting the configuration of a mathematical sign of inequality ">" at the right edge portion of the substrate. In the case where a single thermal head is constituted of a plurality of substrates having a substantially trapezoidal or inverted trapezoidal shape, such electrode patterns would eliminate the need to change the electrode patterns of the substrates depending on the positions, such as to the left, the center, or the right of the thermal head, where the substrates are arranged. The substrates are generally manufactured by separately cutting a plurality of regions previously formed on an insulated substrate and each constituting a substrate. Therefore, if the same electrode patterns are applicable to all the substrates, as in the present invention, the same photo mask can be used to reduce the number of manufacturing steps. Even though a plurality of substrates having a trapezoidal or inverted trapezoidal shape are combined to constitute a single thermal head, the positions where the electrodes are arranged with respect to the paper feed direction will not differ for each of the substrates, thus allowing the print data to be transferred in the same direction. It is therefore not necessary to arrange additional wiring, ensuring the manufacture of thermal heads with a simple structure while keeping a good printing quality. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a structural diagram depicting the entirety of a conventional thermal head; Fig. 2 is an enlarged view showing the major part of the thermal head in Fig. 1; Fig. 3 is a structural diagram depicting the entirety of another thermal head in the related art; Fig. 4 is a structural diagram showing the major part of a thermal head in accordance with an embodiment of the present invention; Fig. 5 is a diagram for explaining cutting planes, in the process of manufacturing, of a substrate in accordance with the present invention; Fig. 6 is a structural diagram depicting the entirety of a thermal head in accordance with an embodiment of the present invention; Fig. 7 is an enlarged top plan view showing the left end of a substrate 20 in Fig. 5; Fig. 8 is an enlarged perspective view showing the left end of a substrate in accordance with another embodiment of the present invention; and Fig. 9 is an enlarged perspective view showing the central portion of the substrate in Fig. 8. DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will now be described with reference to the accompanying drawings. Embodiment 1 Fig. 4 is a diagram depicting the constitution of a thermal head embodying the present invention, in which parts identical to those of thermal heads shown in Figs. 1 and 2 are designated by the same reference numerals. In Fig. 4, substrates 20a and 20c each having a generally trapezoidal shape in plan view and a substrate 20b having an inverted trapezoidal shape are transversely and alternately arranged in the direction orthogonal to the paper feed direction F so as to constitute a single thermal head. On the substrates 20a, 20b, and 20c are belt-like heating elements 16 extending along the long sides of the substrates and in the direction substantially orthogonal to the paper feed direction F. Comb-teeth-shaped common electrodes 12 and individual electrodes 14 are disposed in an alternate manner with respect to the heating elements 16. The heating elements 16 are discontinuous among the substrates 20a, 20b, and 20c. In order to prevent the deterioration of the printing quality, the side edge portions (lines I-I' and II-II' in the figure) of the substrates form predetermined angles 3' and 3 with respect to the paper feed direction F. More specifically, the side edge portions of a substrate adjacent to the other substrates are cut to form supplementary angles other than 90° with respect to one another. The trapezoidal substrates 20a and 20c are arranged relative to the inverted trapezoidal substrates 20b in such a manner that their respective heating elements 16 are displaced from each other by a length L. A specific constitution of the electrode patterns, by way of example, of the inverted trapezoidal substrate 20b will be described hereinbelow. It will be noted that the electrode patterns of the other trapezoidal substrates 20a and 20c are basically the same as that of the substrate 20b. The common electrodes 12 and the individual electrodes 14 provided in the vicinity of the side edge portions of the substrate 20b extend from the heating elements 16 toward the upper and lower end portions (which correspond to the upper and lower sides of the trapezium) in such a manner that they are angled toward the central portion of the substrate 20b. More specifically, the common electrodes 12 disposed at the left end on one side of the heating element 16 of the substrate 20b form a predetermined angle 1 in a clockwise direction with respect to the direction orthogonal to the long sides of the heating element 16, while the common electrodes 12 disposed at the right end form an angle 2' in a counterclockwise direction. These angles 1 and 2' diminish accordingly as approaching the center of the substrate 20b and become substantially zero at the center of the substrate 20b. Although the angles 1 and 2' are to be changed for each of the electrodes in this embodiment, the change may be made collectively for a plurality of electrodes. In contrast with the common electrodes 12, the individual electrodes 14 disposed at the left end on the other side of the substrate 20b form a predetermined angle 1' in the counter-clockwise direction with respect to the direction orthogonal to the long sides of the heating element 16 while the individual electrodes 14 disposed at the right end form an angle 2 in a clockwise direction. These angles 1' and 2 diminish accordingly as the center of the substrate 20b is approached, and become substantially zero at the center of the substrate 20b. The amount of change was set to be Δ for each of the electrodes in the same manner as the amount of the change in the pattern of the common electrodes 12. The angles of the electrode patterns in this embodiment can be ¦1¦ = ¦2'¦ = ¦1'¦ = ¦2¦ or ¦1¦ =¦2¦, ¦1'¦ = ¦2'¦. The values of the angles 3 and 3' of lines I-I' and II-II', respectively, with respect to the paper feed direction F can be 3' = 1', 3 = 2, respectively. As a result, the electrode patterns will present the configuration of a mathematical sign of inequality "<" at the left end of the substrate cut into a desired shape, and the configuration of a sign of inequality ">" at the right end thereof. It is to be appreciated that these angles 1, 1', 2, 2', 3, 3' do not necessarily need to be equal. These angles may have such predetermined magnitudes in the substrate cut into a desired shape such as a trapezium or an inverted trapezium that the electrode patterns of the common electrode and the individual electrodes can be angled toward the central portion of the substrate in the vicinity of the side edge portions of the substrate while extending from the heating element toward the upper and lower ends of the substrate. The magnitudes of the angles 1, 1', 2, 2' in these electrode patterns can be set to desired values depending on the amount of change Δ for each electrode or for a plurality of electrodes and the print dot density of the thermal head. In manufacturing a substrate of trapezoidal shape or inverted trapezoidal shape having such electrode patterns, the heating element 16, common electrodes 12 and individual electrodes 14 are formed in each region constituting each substrate 20 on an insulated substrate (not shown) made of, e.g., alumina ceramic, as illustrated in Fig. 5. The insulated substrate is then cut along the line AC1-AC2 or AC3-AC4 in the diagram to obtain a substrate 20c or 20a, respectively, having the trapezoidal shape shown in Fig. 4. A substrate 20b having the inverted trapezoidal shape in Fig. 4 is obtained by cutting the insulated substrate along the line B1-B2 and B3-B4. Thus, in the case of using the substrates 20a, 20b, and 20c in Fig. 4 to constitute a single thermal head as depicted in Fig. 6, the same electrode patterns can be employed for the substrates 20a, 20b, and 20c having substantially trapezoidal or inverted trapezoidal shape in top plan view, thereby enabling the manufacture of the substrates by use of only one photo mask in the production process. Furthermore, the positions where the electrodes are arranged with respect to the paper feed direction do not differ depending on the substrates 20 having the trapezoidal or inverted trapezoidal shape, thus ensuring the same transfer direction of the print data and eliminating the need for additional wiring even though a plurality of substrates 20 are transversely arranged to constitute a single thermal head. In order to obtain the same effect as the present embodiment, the shape of the substrate 20 formed by cutting the insulated substrate is not limited to the trapezium or inverted trapezium as long as the side edge portions adjacent to the other substrates 20 form a predetermined angle with respect to the paper feed direction. Fig. 7 is an enlarged top plan view of the heating element 16 and its vicinity at the left end of the substrate 20 shown in Fig. 5. Comb-teeth-like common electrodes 12 and individual electrodes 14 are alternately arranged on the substrate, and a resistor layer 15 is superposed on the two types of electrodes. A hatched area 17 designates a heating region in the resistor layer 15 which generates heat by the individual electrodes 14 and the common electrodes adjoining thereto. In this embodiment, the belt-like heating element 16 consists of a plurality of aligned heating regions 17. Actually, an insulated protective film not shown herein is further superposed thereon so as to cover the whole of the substrate. The electrode patterns are so formed that the common electrodes 12 and individual electrodes 14 located near the side edge portions of the substrate and extending from the heating element 16 toward the upper and lower ends of the substrate are angled toward the central portion of the substrate. More specifically, the common electrodes 12 disposed on one side of the heating element 16 form a predetermined angle in a clockwise direction with respect to the direction orthogonal to the long sides of the heating element 16, whereas the parts of the common electrodes 12 extending from the other side of the heating element 16 form an angle 1' in a counterclockwise direction. On the contrary, the individual electrodes 14 disposed on the other side of the heating element 16 form an angle 1' in a counterclockwise direction, whereas the parts of the individual electrodes 14 extending from the one side of the heating element on which the common electrodes are mounted form a predetermined angle 1 in a clockwise direction. Therefore, at the left end of the substrate being cut into a desired shape such as trapezium or inverted trapezium, the electrode patterns of the common electrodes 12 and the individual electrodes 14 have the configuration of the sign of inequality "<". On the other hand, at the right end not shown of the substrate, the common electrodes 12 disposed on the one side of the heating element 16 form a predetermined angle 1' in a counterclockwise direction with respect to the direction orthogonal to the long sides of the heating element 16, whereas the parts of the common electrodes 12 extending from the other side of the heating element 16 form a predetermined angle 2 in a clockwise direction. On the contrary, the individual electrodes 14 disposed on the other side of the heating element 16 form a predetermined angle 2 in a clockwise direction whereas the parts of the individual electrodes 14 extending from the one side of the heating element 16 on which the common electrodes 12 are arranged to form a predetermined angle 2' in counterclockwise direction. Therefore, at the right end of the substrate, the electrode patterns of the common electrodes 12 and the individual electrodes 14 present a configuration of the sign of inequality ">". Correspondingly, at the left end of the substrate the heating regions 17 substantially have a configuration of the sign of inequality "<" as shown in Fig. 4, while at the right end not shown of the substrate the heating regions 17 substantially have a configuration of the sign of inequality ">". At the central portion of the substrate, the common electrodes 12 and the individual electrodes 14 alternately disposed along the heating element 16 are substantially orthogonal to the heating element 16. As a result, at the central portion of the substrate, the heating regions 17 exhibit a configuration of a quadrangle. The heating regions 17 in this embodiment have different configurations and areas between the side edge portions of the substrate and the central portion of the substrate. However, the angle of the electrode patterns with respect to the heating element 16 comes near to a right angle accordingly as the central portion of the substrate is approached so that there is very little difference in configuration and area between two heating regions 17 adjacent to each other. Moreover, the areas of the heating regions 17 are not so large, compared with the resolution of the human eye. Accordingly, the above will not cause any practical problems in printing. In the case of using a plurality of substrates to constitute a single thermal head as shown in Fig. 6, as described hereinbefore, the constitution of this embodiment allows the plurality of substrates to be constituted with the same electrode patterns. Since the positions of the electrodes arranged with respect to the paper feed direction do not differ among the substrates having a trapezoidal shape and an inverted trapezoidal shape, the thermal head consisting of a plurality of substrates allows print data to be transferred in the same direction, which will eliminate the necessity of arranging additional wiring. Embodiment 2 Referring to Fig. 8, another embodiment of the present invention will be described. Fig. 8 is an enlarged perspective view showing the left end of a substrate in the case of using a thermal head element different from the first embodiment. On an insulated substrate 23 made of, e.g., alumina ceramic, a belt-like glazed layer 25 is formed which is spanned with resistor layers 28 and then common electrodes 22 and individual electrodes 24 are each superposed on the resistor layers 28. An insulated protective film not shown is actually laminated thereon so as to cover the entirety of the substrate. The common electrodes 22 and the individual electrodes 24 are separated from each other and formed by selectively etching, on a part of the glazed layer, electrically conductive layers which have been formed on the resistor layers 28. The portions of the resistor layers 28 exposed by etching (hatched portions in the figure) define heating regions 27 which are controlled by the common electrodes 22 and individual electrodes 24 confronting each other. Assuming that a row of the heating regions 27 is a heating element 26, the common electrodes form a predetermined angle 1 in a clockwise direction with respect to the direction orthogonal to the long side of the heating element 26, while the individual electrodes 22 are angled at 1' in a counterclockwise direction. Consequently, the heating regions 27 will substantially present a configuration of the sign of inequality "<". On the other hand, at the right end not shown of the substrate having in plan view a trapezoidal or inverted trapezoidal shape, the common electrodes 22 are disposed so as to form a predetermined angle 2' in a counterclockwise direction with respect to the direction orthogonal to the heating element 26, while the individual electrodes 24 are angled at 2 in a clockwise direction. Therefore, the heating regions 27 will present a configuration of the sign of inequality ">" which is contrary to Fig. 8. Fig. 9 is an enlarged perspective view of the central portion of the substrate depicted in Fig. 8. Elements identical to those in Fig. 8 are designated by the same reference numerals, which will not be further described. At the central portion of the substrate, the common electrodes 22 and the individual electrodes 24 confronting each other are arranged so as to form substantially right angles, respectively, with respect to the heating element 26 so that the heating regions can present a configuration of the quadrangle, without bending, on the glazed layer 25. It is to be appreciated that the angles 1, 1', 2, 2' decrease accordingly as the center of the substrate is approached and results in nearly zero at the center of the substrate, and that the amount of change in angle is set at Δ for each of the electrodes in this embodiment, which will not be limited to this in the same manner as the first embodiment. As shown in Figs. 8 and 9, the heating regions 27 in this embodiment have different configurations and areas between the side edge portions of the substrate and the central portion of the substrate. However, there is very little difference in configuration and area between the two heating regions 27 adjacent to each other. Moreover, the areas of the heating regions 27 are not so large, compared with the resolution of the human eye. Accordingly, the above will not cause any practical problems in printing. In the case of using a plurality of substrates to constitute a single thermal head as shown in Fig. 6, as in the first embodiment described above, this embodiment will allow the plurality of substrates to be constituted with the same. electrode patterns. Since the positions of the electrodes arranged with respect to the paper feed direction do not differ among the substrates having a trapezoidal shape and an inverted trapezoidal shape, the thermal head consisting of a plurality of substrates allows print data to be transferred in the same direction, which will eliminate the necessity of arranging additional wiring. Although a thermal head consisting of a plurality of substrates cut into desired shapes has been described in the first and second embodiment, the constitution of the present invention is also applicable to a thermal head consisting of a single substrate. According to a thermal head having electrode patterns in accordance with the present invention, as described above, it is possible to form a plurality of substrates with the same electrode patterns when constituting a single thermal head by arranging the plurality of substrates. It is also possible to provide a thermal head in which all substrates have the same data transfer direction to simplify the arrangement of wiring.

Claims: A thermal head consisting of at least two or more substrates (20a, 20b, 20c) arranged in a transverse row and each having in plan view a substantially trapezoidal shape, each of said substrates (20a, 20b, 20c) comprising: a heating element (16, 26) formed on said substrate (20a, 20b, 20c) in a belt or in a row along the longitudinal direction of said substrate (20a, 20b, 20c) defined by the direction of the substantially parallel sides of said trapezoidal substrate (20a, 20b, 20c); a plurality of common electrodes (12, 22) mainly arranged on one side along the longitudinal direction of said heating element (16, 26); and a plurality of individual electrodes (14, 24) mainly arranged on the other side of said heating element (16, 26); wherein said two types of electrodes (12, 22, 14, 24) positioned in the vicinity of the edges of said substrate corresponding to the oblique sides of said trapezium extend from said heating element (16, 26) toward the edges of said substrate (20a, 20b, 20c) corresponding to the parallel sides of said trapezium in such a manner as to be directed at an angle (1, 2'; 1', 2) toward the central portion of said substrate (20a, 20b, 20c). A thermal head according to claim 1, wherein electrode patterns of said common electrodes (12, (22) and said individual electrodes (14, 24) are substantially orthogonal to the longitudinal direction of said heating element (16, 26) at the central portion of said substrate. A thermal head according to claim 1, wherein the angle of said two types of electrodes (12, 14; 22, 24) diminishes accordingly as the central portion of said substrate is approached from the edges of said substrate corresponding to the oblique sides of the trapezium. A thermal head according to claim 1, wherein said common electrodes (12, 22) and said individual electrodes (14, 24) are alternately arranged at the portion where said heating element (16, 26) is formed. A thermal head according to claim 1, wherein said common electrodes (12, 22) and said individual electrodes (14, 24) confront each other at the portion where said heating element (16, 26) is formed. A thermal head according to claim 1, wherein the edges of said substrate corresponding to the oblique sides of said trapezium are formed at angles making supplementary angles other than 90 degrees with said edges of another substrate adjoining to said substrate. A thermal head according to claim 6, wherein the angles of the edges of said substrate corresponding to the oblique sides of said trapezium with respect to the longitudinal direction of said substrate are substantially equal to the angles of one electrode of said two types of electrodes disposed on the edges of said substrate with respect to said longitudinal direction. A thermal head according to claim 1, wherein heating regions (17, 27) of said heating element (16, 26) formed in the vicinity of the edges of said substrate corresponding to the oblique sides of said trapezium present the configuration resembling a sign of inequality "<" or ">". A thermal head according to claim 8, wherein the heating regions (17, 27) of said heating element (16, 26) formed at the central portion of said substrate present the configuration resembling a quadrangle.

Inventors: AKASAKA HIDEYA, TAGASHIRA FUMIAKI, AKASAKA, HIDEYA, TAGASHIRA, FUMIAKI
Applicants: ROHM CO LTD, ROHM CO., LTD.
Application year: 1994
Main classification: B41J2
IPC codes: B41J2